Some contractors skip basic steps in building wood retaining walls. This information will help you to insure your wall will not only look good, but last a lifetime.
As an engineer, I have been asked to investigate and identify the cause of failure of wood retaining walls.
I would like for us to walk together through an investigation of a timber retaining wall. The owner asked me to inspect his wall and to tell him why his retaining wall was failing. He had plans to replace the wall and wanted his new wall to last a long time. Also, see wood retaining wall.
Let's look at his treated timber retaining wall and determine why it failed. His wall had a number of issues that contributed to its failure.
It should be noted that timber retaining walls have a limited life. But a timber wall, built correctly, should last more than 20 years.
Timber walls are very popular with homeowners. They are the easiest type of wall to build and they are most often the most economical. They fit into formal as well as casual landscape environments.
One of the first things I look for are the number of deadmen in the wall. Deadmen are timbers that extend back into the embankment to give strength to the wall. You will only see the end of the timbers in the face of the wall.
Look at the photograph. Your first reaction may be like mine. It's age! This wall failed because of its age and rotted!
As you can see from the photograph, there are few deadmen in this wall. I think we can conclude that the lack of a sufficient number of deadmen contributed to the wall's failure. Count the number of deadmen in the face of the wall shown in the photograph... only a few deadmen were used in this wall.
Without sufficient deadmen, the wall can move. It will move outward. When the wall moves outward, space is created behind the wall. Rain water will get into this space.
The strength and stability of retaining walls come from the deadmen anchor. The number of deadmen required depends on the height of the retaining wall. The height of this wall is more than seven feet. The higher the wall the more deadmen needed.
The type of soil behind the wall affects the number of deadmen required. It is always prudent to retain an engineer to evaluate the soil and advise you of the length and number of deadmen needed for a particular project.
Most municipalities require a permit for timber retaining walls over 5 feet in height and require the wall to be designed by a professional engineer. See Building Retaining Walls.
Second, hydrostatic pressure builds up behind a wall when water is trapped behind the wall. This is one of the leading causes for retaining wall failures.
When water soaks into the ground behind a retaining wall and has no place to go, the pressure behind the wall increases. It will eventually exert a tremendous force on the wall and cause the wall to bow outward and collapse. As you can see in the photograph, water pressure has pushed timbers outward.
A good design will include granular rock and a perforated pipe behind the wall to collect the water and drain it away from the wall. The removal of the water behind the wall prevents hydrostatic pressure from developing and exerting hydrostatic pressure forces on the timbers.
My inspection determined that this wall did not have a drainage system behind the wall. Hydrostatic pressure developed and pushed the wall forward. Note the bulge in the retaining wall in the photograph.
Third, this wall was constructed to trap water on the surface behind the wall. The wood retaining wall seen in the photograph shows the sunken area behind the wall. Water was able to pool on the ground and slowly soaked into the soil. This increased the wet cycle and shortened the life of wood wall.
Often, backfill is not compacted sufficiently and the ground settles. As the ground settles, ground surface pockets are created which hold water. The water soaks into the ground behind the wall. The unsuspecting homeowner does not realize the contractor has left a serious problem for him to deal with in the future.
I highly recommend installing a drainage ditch immediately behind the wall to catch the surface water. The ditch should be sloped to drain water to a collection point and discharged.
You can build your wood retaining wall. Select your contractor with care. Check him out and find someone skilled in building wood retaining walls.